Using 2 cpu threads in CI runner

I am hosting a project on GitLab.com where I am running multiple test jobs. It happens that they are not detecting an issue which is later caught by a platform when software is being publish. The problem comes from the fact that GitLab.com shared runners have only 1 cpu thread available and the issue manifest when there are 2+ cpu threads involved.

Is there any way I could instruct my gitlab ci yaml file so that it will try to get at least 2 cores on a runner machine?
Or the only way is to have specific private runner?

Alternatively, is there any other place when I can submit my question? or requests for such functionality?